.container {
  height: 200px;
  background-color: lightgreen;
}
.box_3 {
  height: 100px;
  padding-top: 0;
  background-color: lightpink;
  animation: p_top 5s infinite;
  
}

@keyframes p_top {
  50% {
    padding-top: 50px;
  }
}
